Merge host.endpoint into host.ConnectedEndpoint

host.endpoint contained duplicated logic from the sockerpair implementation and
host.ConnectedEndpoint. Remove host.endpoint in favor of a
host.ConnectedEndpoint wrapped in a socketpair end.

PiperOrigin-RevId: 217240096
Change-Id: I4a3d51e3fe82bdf30e2d0152458b8499ab4c987c

9 files changed, 134 insertions, 250 deletions

diff --git a/pkg/sentry/context/contexttest/contexttest.go b/pkg/sentry/context/contexttest/contexttest.go
index 193ce3440..b3c6a566b 100644
--- a/pkg/sentry/context/contexttest/contexttest.go
+++ b/pkg/sentry/context/contexttest/contexttest.go
@@ -53,6 +53,14 @@ type testContext struct {
 // globalUniqueID tracks incremental unique identifiers for tests.
 var globalUniqueID uint64
 
+// globalUniqueIDProvider implements unix.UniqueIDProvider.
+type globalUniqueIDProvider struct{}
+
+// UniqueID implements unix.UniqueIDProvider.UniqueID.
+func (*globalUniqueIDProvider) UniqueID() uint64 {
+	return atomic.AddUint64(&globalUniqueID, 1)
+}
+
 // lastInotifyCookie is a monotonically increasing counter for generating unique
 // inotify cookies. Must be accessed using atomic ops.
 var lastInotifyCookie uint32
@@ -76,7 +84,9 @@ func (t *testContext) Value(key interface{}) interface{} {
 	case platform.CtxPlatform:
 		return t.platform
 	case uniqueid.CtxGlobalUniqueID:
-		return atomic.AddUint64(&globalUniqueID, 1)
+		return (*globalUniqueIDProvider).UniqueID(nil)
+	case uniqueid.CtxGlobalUniqueIDProvider:
+		return &globalUniqueIDProvider{}
 	case uniqueid.CtxInotifyCookie:
 		return atomic.AddUint32(&lastInotifyCookie, 1)
 	case ktime.CtxRealtimeClock:
diff --git a/pkg/sentry/fs/host/BUILD b/pkg/sentry/fs/host/BUILD
index 6d5640f0a..5ada32ee1 100644
--- a/pkg/sentry/fs/host/BUILD
+++ b/pkg/sentry/fs/host/BUILD
@@ -42,7 +42,9 @@ go_library(
         "//pkg/sentry/safemem",
         "//pkg/sentry/socket/control",
         "//pkg/sentry/socket/unix",
+        "//pkg/sentry/uniqueid",
         "//pkg/sentry/usermem",
+        "//pkg/syserr",
         "//pkg/syserror",
         "//pkg/tcpip",
         "//pkg/tcpip/link/rawfile",
diff --git a/pkg/sentry/fs/host/socket.go b/pkg/sentry/fs/host/socket.go
index 68ebf6402..577e9e272 100644
--- a/pkg/sentry/fs/host/socket.go
+++ b/pkg/sentry/fs/host/socket.go
@@ -25,6 +25,8 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/socket/control"
 	unixsocket "gvisor.googlesource.com/gvisor/pkg/sentry/socket/unix"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/uniqueid"
+	"gvisor.googlesource.com/gvisor/pkg/syserr"
 	"gvisor.googlesource.com/gvisor/pkg/syserror"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/link/rawfile"
@@ -39,88 +41,35 @@ import (
 // N.B. 8MB is the default maximum on Linux (2 * sysctl_wmem_max).
 const maxSendBufferSize = 8 << 20
 
-// endpoint encapsulates the state needed to represent a host Unix socket.
-//
-// TODO: Remove/merge with ConnectedEndpoint.
-//
-// +stateify savable
-type endpoint struct {
-	queue waiter.Queue `state:"zerovalue"`
-
-	// fd is the host fd backing this file.
-	fd int `state:"nosave"`
-
-	// If srfd >= 0, it is the host fd that fd was imported from.
-	srfd int `state:"wait"`
-
-	// stype is the type of Unix socket.
-	stype unix.SockType `state:"nosave"`
-
-	// sndbuf is the size of the send buffer.
-	sndbuf int `state:"nosave"`
-}
-
-func (e *endpoint) init() error {
-	family, err := syscall.GetsockoptInt(e.fd, syscall.SOL_SOCKET, syscall.SO_DOMAIN)
-	if err != nil {
-		return err
-	}
-
-	if family != syscall.AF_UNIX {
-		// We only allow Unix sockets.
-		return syserror.EINVAL
-	}
-
-	stype, err := syscall.GetsockoptInt(e.fd, syscall.SOL_SOCKET, syscall.SO_TYPE)
-	if err != nil {
-		return err
-	}
-	e.stype = unix.SockType(stype)
-
-	e.sndbuf, err = syscall.GetsockoptInt(e.fd, syscall.SOL_SOCKET, syscall.SO_SNDBUF)
-	if err != nil {
-		return err
-	}
-	if e.sndbuf > maxSendBufferSize {
-		log.Warningf("Socket send buffer too large: %d", e.sndbuf)
-		return syserror.EINVAL
-	}
-
-	if err := syscall.SetNonblock(e.fd, true); err != nil {
-		return err
-	}
-
-	return fdnotifier.AddFD(int32(e.fd), &e.queue)
-}
-
-// newEndpoint creates a new host endpoint.
-func newEndpoint(fd int, srfd int) (*endpoint, error) {
-	ep := &endpoint{fd: fd, srfd: srfd}
-	if err := ep.init(); err != nil {
-		return nil, err
-	}
-	return ep, nil
-}
-
 // newSocket allocates a new unix socket with host endpoint.
-func newSocket(ctx context.Context, fd int, saveable bool) (*fs.File, error) {
-	ownedfd := fd
+func newSocket(ctx context.Context, orgfd int, saveable bool) (*fs.File, error) {
+	ownedfd := orgfd
 	srfd := -1
 	if saveable {
 		var err error
-		ownedfd, err = syscall.Dup(fd)
+		ownedfd, err = syscall.Dup(orgfd)
 		if err != nil {
 			return nil, err
 		}
-		srfd = fd
+		srfd = orgfd
 	}
-	ep, err := newEndpoint(ownedfd, srfd)
+	f := fd.New(ownedfd)
+	var q waiter.Queue
+	e, err := NewConnectedEndpoint(f, &q, "" /* path */)
 	if err != nil {
 		if saveable {
-			syscall.Close(ownedfd)
+			f.Close()
+		} else {
+			f.Release()
 		}
-		return nil, err
+		return nil, syserr.TranslateNetstackError(err).ToError()
 	}
+
+	e.srfd = srfd
+	e.Init()
+
+	ep := unix.NewExternal(e.stype, uniqueid.GlobalProviderFromContext(ctx), &q, e, e)
+
 	return unixsocket.New(ctx, ep), nil
 }
 
@@ -130,139 +79,22 @@ func newSocket(ctx context.Context, fd int, saveable bool) (*fs.File, error) {
 //
 // NewSocketWithDirent takes ownership of f on success.
 func NewSocketWithDirent(ctx context.Context, d *fs.Dirent, f *fd.FD, flags fs.FileFlags) (*fs.File, error) {
-	ep, err := newEndpoint(f.FD(), -1)
+	f2 := fd.New(f.FD())
+	var q waiter.Queue
+	e, err := NewConnectedEndpoint(f2, &q, "" /* path */)
 	if err != nil {
-		return nil, err
+		f2.Release()
+		return nil, syserr.TranslateNetstackError(err).ToError()
 	}
 
 	// Take ownship of the FD.
 	f.Release()
 
-	return unixsocket.NewWithDirent(ctx, d, ep, flags), nil
-}
-
-// Close implements unix.Endpoint.Close.
-func (e *endpoint) Close() {
-	fdnotifier.RemoveFD(int32(e.fd))
-	syscall.Close(e.fd)
-	e.fd = -1
-}
-
-// EventRegister implements waiter.Waitable.EventRegister.
-func (e *endpoint) EventRegister(we *waiter.Entry, mask waiter.EventMask) {
-	e.queue.EventRegister(we, mask)
-	fdnotifier.UpdateFD(int32(e.fd))
-}
-
-// EventUnregister implements waiter.Waitable.EventUnregister.
-func (e *endpoint) EventUnregister(we *waiter.Entry) {
-	e.queue.EventUnregister(we)
-	fdnotifier.UpdateFD(int32(e.fd))
-}
-
-// Readiness implements unix.Endpoint.Readiness.
-func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
-	return fdnotifier.NonBlockingPoll(int32(e.fd), mask)
-}
-
-// Type implements unix.Endpoint.Type.
-func (e *endpoint) Type() unix.SockType {
-	return e.stype
-}
-
-// Connect implements unix.Endpoint.Connect.
-func (e *endpoint) Connect(server unix.BoundEndpoint) *tcpip.Error {
-	return tcpip.ErrInvalidEndpointState
-}
-
-// Bind implements unix.Endpoint.Bind.
-func (e *endpoint) Bind(address tcpip.FullAddress, commit func() *tcpip.Error) *tcpip.Error {
-	return tcpip.ErrInvalidEndpointState
-}
-
-// Listen implements unix.Endpoint.Listen.
-func (e *endpoint) Listen(backlog int) *tcpip.Error {
-	return tcpip.ErrInvalidEndpointState
-}
-
-// Accept implements unix.Endpoint.Accept.
-func (e *endpoint) Accept() (unix.Endpoint, *tcpip.Error) {
-	return nil, tcpip.ErrInvalidEndpointState
-}
-
-// Shutdown implements unix.Endpoint.Shutdown.
-func (e *endpoint) Shutdown(flags tcpip.ShutdownFlags) *tcpip.Error {
-	return tcpip.ErrInvalidEndpointState
-}
-
-// GetSockOpt implements unix.Endpoint.GetSockOpt.
-func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
-	switch o := opt.(type) {
-	case tcpip.ErrorOption:
-		_, err := syscall.GetsockoptInt(e.fd, syscall.SOL_SOCKET, syscall.SO_ERROR)
-		return translateError(err)
-	case *tcpip.PasscredOption:
-		// We don't support passcred on host sockets.
-		*o = 0
-		return nil
-	case *tcpip.SendBufferSizeOption:
-		*o = tcpip.SendBufferSizeOption(e.sndbuf)
-		return nil
-	case *tcpip.ReceiveBufferSizeOption:
-		// N.B. Unix sockets don't use the receive buffer. We'll claim it is
-		// the same size as the send buffer.
-		*o = tcpip.ReceiveBufferSizeOption(e.sndbuf)
-		return nil
-	case *tcpip.ReuseAddressOption:
-		v, err := syscall.GetsockoptInt(e.fd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR)
-		*o = tcpip.ReuseAddressOption(v)
-		return translateError(err)
-	case *tcpip.ReceiveQueueSizeOption:
-		return tcpip.ErrQueueSizeNotSupported
-	}
-	return tcpip.ErrInvalidEndpointState
-}
-
-// SetSockOpt implements unix.Endpoint.SetSockOpt.
-func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
-	return nil
-}
-
-// GetLocalAddress implements unix.Endpoint.GetLocalAddress.
-func (e *endpoint) GetLocalAddress() (tcpip.FullAddress, *tcpip.Error) {
-	return tcpip.FullAddress{}, nil
-}
-
-// GetRemoteAddress implements unix.Endpoint.GetRemoteAddress.
-func (e *endpoint) GetRemoteAddress() (tcpip.FullAddress, *tcpip.Error) {
-	return tcpip.FullAddress{}, nil
-}
-
-// Passcred returns whether or not the SO_PASSCRED socket option is
-// enabled on this end.
-func (e *endpoint) Passcred() bool {
-	// We don't support credential passing for host sockets.
-	return false
-}
-
-// ConnectedPasscred returns whether or not the SO_PASSCRED socket option
-// is enabled on the connected end.
-func (e *endpoint) ConnectedPasscred() bool {
-	// We don't support credential passing for host sockets.
-	return false
-}
+	e.Init()
 
-// SendMsg implements unix.Endpoint.SendMsg.
-func (e *endpoint) SendMsg(data [][]byte, controlMessages unix.ControlMessages, to unix.BoundEndpoint) (uintptr, *tcpip.Error) {
-	if to != nil {
-		return 0, tcpip.ErrInvalidEndpointState
-	}
-
-	// Since stream sockets don't preserve message boundaries, we can write
-	// only as much of the message as fits in the send buffer.
-	truncate := e.stype == unix.SockStream
+	ep := unix.NewExternal(e.stype, uniqueid.GlobalProviderFromContext(ctx), &q, e, e)
 
-	return sendMsg(e.fd, data, controlMessages, e.sndbuf, truncate)
+	return unixsocket.NewWithDirent(ctx, d, ep, flags), nil
 }
 
 func sendMsg(fd int, data [][]byte, controlMessages unix.ControlMessages, maxlen int, truncate bool) (uintptr, *tcpip.Error) {
@@ -278,18 +110,6 @@ func sendMsg(fd int, data [][]byte, controlMessages unix.ControlMessages, maxlen
 	return n, translateError(err)
 }
 
-// RecvMsg implements unix.Endpoint.RecvMsg.
-func (e *endpoint) RecvMsg(data [][]byte, creds bool, numRights uintptr, peek bool, addr *tcpip.FullAddress) (uintptr, uintptr, unix.ControlMessages, *tcpip.Error) {
-	// N.B. Unix sockets don't have a receive buffer, the send buffer
-	// serves both purposes.
-	rl, ml, cm, err := recvMsg(e.fd, data, numRights, peek, addr, e.sndbuf)
-	if rl > 0 && err == tcpip.ErrWouldBlock {
-		// Message did not fill buffer; that's fine, no need to block.
-		err = nil
-	}
-	return rl, ml, cm, err
-}
-
 func recvMsg(fd int, data [][]byte, numRights uintptr, peek bool, addr *tcpip.FullAddress, maxlen int) (uintptr, uintptr, unix.ControlMessages, *tcpip.Error) {
 	var cm unet.ControlMessage
 	if numRights > 0 {
@@ -328,42 +148,21 @@ func recvMsg(fd int, data [][]byte, numRights uintptr, peek bool, addr *tcpip.Fu
 // be called twice because host.ConnectedEndpoint is both a unix.Receiver and
 // unix.ConnectedEndpoint.
 func NewConnectedEndpoint(file *fd.FD, queue *waiter.Queue, path string) (*ConnectedEndpoint, *tcpip.Error) {
-	family, err := syscall.GetsockoptInt(file.FD(), syscall.SOL_SOCKET, syscall.SO_DOMAIN)
-	if err != nil {
-		return nil, translateError(err)
-	}
-
-	if family != syscall.AF_UNIX {
-		// We only allow Unix sockets.
-		return nil, tcpip.ErrInvalidEndpointState
-	}
-
-	stype, err := syscall.GetsockoptInt(file.FD(), syscall.SOL_SOCKET, syscall.SO_TYPE)
-	if err != nil {
-		return nil, translateError(err)
-	}
-
-	sndbuf, err := syscall.GetsockoptInt(file.FD(), syscall.SOL_SOCKET, syscall.SO_SNDBUF)
-	if err != nil {
-		return nil, translateError(err)
-	}
-	if sndbuf > maxSendBufferSize {
-		log.Warningf("Socket send buffer too large: %d", sndbuf)
-		return nil, tcpip.ErrInvalidEndpointState
+	e := ConnectedEndpoint{
+		path:  path,
+		queue: queue,
+		file:  file,
+		srfd:  -1,
 	}
 
-	e := &ConnectedEndpoint{
-		path:   path,
-		queue:  queue,
-		file:   file,
-		stype:  unix.SockType(stype),
-		sndbuf: sndbuf,
+	if err := e.init(); err != nil {
+		return nil, err
 	}
 
 	// AtomicRefCounters start off with a single reference. We need two.
 	e.ref.IncRef()
 
-	return e, nil
+	return &e, nil
 }
 
 // Init will do initialization required without holding other locks.
@@ -376,7 +175,7 @@ func (c *ConnectedEndpoint) Init() {
 // ConnectedEndpoint is a host FD backed implementation of
 // unix.ConnectedEndpoint and unix.Receiver.
 //
-// ConnectedEndpoint does not support save/restore for now.
+// +stateify savable
 type ConnectedEndpoint struct {
 	queue *waiter.Queue
 	path  string
@@ -385,11 +184,11 @@ type ConnectedEndpoint struct {
 	ref refs.AtomicRefCount
 
 	// mu protects fd, readClosed and writeClosed.
-	mu sync.RWMutex
+	mu sync.RWMutex `state:"nosave"`
 
 	// file is an *fd.FD containing the FD backing this endpoint. It must be
 	// set to nil if it has been closed.
-	file *fd.FD
+	file *fd.FD `state:"nosave"`
 
 	// readClosed is true if the FD has read shutdown or if it has been closed.
 	readClosed bool
@@ -398,6 +197,9 @@ type ConnectedEndpoint struct {
 	// closed.
 	writeClosed bool
 
+	// If srfd >= 0, it is the host FD that file was imported from.
+	srfd int `state:"wait"`
+
 	// stype is the type of Unix socket.
 	stype unix.SockType
 
@@ -407,7 +209,44 @@ type ConnectedEndpoint struct {
 	// GetSockOpt and message splitting/rejection in SendMsg, but do not
 	// prevent lots of small messages from filling the real send buffer
 	// size on the host.
-	sndbuf int
+	sndbuf int `state:"nosave"`
+}
+
+// init performs initialization required for creating new ConnectedEndpoints and
+// for restoring them.
+func (c *ConnectedEndpoint) init() *tcpip.Error {
+	family, err := syscall.GetsockoptInt(c.file.FD(), syscall.SOL_SOCKET, syscall.SO_DOMAIN)
+	if err != nil {
+		return translateError(err)
+	}
+
+	if family != syscall.AF_UNIX {
+		// We only allow Unix sockets.
+		return tcpip.ErrInvalidEndpointState
+	}
+
+	stype, err := syscall.GetsockoptInt(c.file.FD(), syscall.SOL_SOCKET, syscall.SO_TYPE)
+	if err != nil {
+		return translateError(err)
+	}
+
+	if err := syscall.SetNonblock(c.file.FD(), true); err != nil {
+		return translateError(err)
+	}
+
+	sndbuf, err := syscall.GetsockoptInt(c.file.FD(), syscall.SOL_SOCKET, syscall.SO_SNDBUF)
+	if err != nil {
+		return translateError(err)
+	}
+	if sndbuf > maxSendBufferSize {
+		log.Warningf("Socket send buffer too large: %d", sndbuf)
+		return tcpip.ErrInvalidEndpointState
+	}
+
+	c.stype = unix.SockType(stype)
+	c.sndbuf = sndbuf
+
+	return nil
 }
 
 // Send implements unix.ConnectedEndpoint.Send.
diff --git a/pkg/sentry/fs/host/socket_state.go b/pkg/sentry/fs/host/socket_state.go
index 6acabd55a..7fa500bfb 100644
--- a/pkg/sentry/fs/host/socket_state.go
+++ b/pkg/sentry/fs/host/socket_state.go
@@ -17,23 +17,26 @@ package host
 import (
 	"fmt"
 	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/fd"
 )
 
 // beforeSave is invoked by stateify.
-func (ep *endpoint) beforeSave() {
-	if ep.srfd < 0 {
+func (c *ConnectedEndpoint) beforeSave() {
+	if c.srfd < 0 {
 		panic("only host file descriptors provided at sentry startup can be saved")
 	}
 }
 
 // afterLoad is invoked by stateify.
-func (ep *endpoint) afterLoad() {
-	fd, err := syscall.Dup(ep.srfd)
+func (c *ConnectedEndpoint) afterLoad() {
+	f, err := syscall.Dup(c.srfd)
 	if err != nil {
-		panic(fmt.Sprintf("failed to dup restored fd %d: %v", ep.srfd, err))
+		panic(fmt.Sprintf("failed to dup restored FD %d: %v", c.srfd, err))
 	}
-	ep.fd = fd
-	if err := ep.init(); err != nil {
-		panic(fmt.Sprintf("Could not restore host socket fd %d: %v", ep.srfd, err))
+	c.file = fd.New(f)
+	if err := c.init(); err != nil {
+		panic(fmt.Sprintf("Could not restore host socket FD %d: %v", c.srfd, err))
 	}
+	c.Init()
 }
diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 238fd127b..cc664deec 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -591,6 +591,8 @@ func (ctx *createProcessContext) Value(key interface{}) interface{} {
 		return ctx.k
 	case uniqueid.CtxGlobalUniqueID:
 		return ctx.k.UniqueID()
+	case uniqueid.CtxGlobalUniqueIDProvider:
+		return ctx.k
 	case uniqueid.CtxInotifyCookie:
 		return ctx.k.GenerateInotifyCookie()
 	default:
@@ -1045,6 +1047,8 @@ func (ctx supervisorContext) Value(key interface{}) interface{} {
 		return ctx.k
 	case uniqueid.CtxGlobalUniqueID:
 		return ctx.k.UniqueID()
+	case uniqueid.CtxGlobalUniqueIDProvider:
+		return ctx.k
 	case uniqueid.CtxInotifyCookie:
 		return ctx.k.GenerateInotifyCookie()
 	default:
diff --git a/pkg/sentry/kernel/task.go b/pkg/sentry/kernel/task.go
index 07ad1614c..4f0b7fe3f 100644
--- a/pkg/sentry/kernel/task.go
+++ b/pkg/sentry/kernel/task.go
@@ -590,6 +590,8 @@ func (t *Task) Value(key interface{}) interface{} {
 		return t.k
 	case uniqueid.CtxGlobalUniqueID:
 		return t.k.UniqueID()
+	case uniqueid.CtxGlobalUniqueIDProvider:
+		return t.k
 	case uniqueid.CtxInotifyCookie:
 		return t.k.GenerateInotifyCookie()
 	default:
diff --git a/pkg/sentry/uniqueid/BUILD b/pkg/sentry/uniqueid/BUILD
index 8eba3609e..ff50b9925 100644
--- a/pkg/sentry/uniqueid/BUILD
+++ b/pkg/sentry/uniqueid/BUILD
@@ -7,5 +7,8 @@ go_library(
     srcs = ["context.go"],
     importpath = "gvisor.googlesource.com/gvisor/pkg/sentry/uniqueid",
     visibility = ["//pkg/sentry:internal"],
-    deps = ["//pkg/sentry/context"],
+    deps = [
+        "//pkg/sentry/context",
+        "//pkg/tcpip/transport/unix",
+    ],
 )
diff --git a/pkg/sentry/uniqueid/context.go b/pkg/sentry/uniqueid/context.go
index eeb8c4286..541e0611d 100644
--- a/pkg/sentry/uniqueid/context.go
+++ b/pkg/sentry/uniqueid/context.go
@@ -18,6 +18,7 @@ package uniqueid
 
 import (
 	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/transport/unix"
 )
 
 // contextID is the kernel package's type for context.Context.Value keys.
@@ -28,6 +29,10 @@ const (
 	// unique identifier.
 	CtxGlobalUniqueID contextID = iota
 
+	// CtxGlobalUniqueIDProvider is a Context.Value key for a
+	// system-wide unique identifier generator.
+	CtxGlobalUniqueIDProvider
+
 	// CtxInotifyCookie is a Context.Value key for a unique inotify
 	// event cookie.
 	CtxInotifyCookie
@@ -38,6 +43,11 @@ func GlobalFromContext(ctx context.Context) uint64 {
 	return ctx.Value(CtxGlobalUniqueID).(uint64)
 }
 
+// GlobalProviderFromContext returns a system-wide unique identifier from ctx.
+func GlobalProviderFromContext(ctx context.Context) unix.UniqueIDProvider {
+	return ctx.Value(CtxGlobalUniqueIDProvider).(unix.UniqueIDProvider)
+}
+
 // InotifyCookie generates a unique inotify event cookie from ctx.
 func InotifyCookie(ctx context.Context) uint32 {
 	return ctx.Value(CtxInotifyCookie).(uint32)
diff --git a/pkg/tcpip/transport/unix/connectioned.go b/pkg/tcpip/transport/unix/connectioned.go
index dd7c03cf1..e319b3bb8 100644
--- a/pkg/tcpip/transport/unix/connectioned.go
+++ b/pkg/tcpip/transport/unix/connectioned.go
@@ -158,6 +158,17 @@ func NewPair(stype SockType, uid UniqueIDProvider) (Endpoint, Endpoint) {
 	return a, b
 }
 
+// NewExternal creates a new externally backed Endpoint. It behaves like a
+// socketpair.
+func NewExternal(stype SockType, uid UniqueIDProvider, queue *waiter.Queue, receiver Receiver, connected ConnectedEndpoint) Endpoint {
+	return &connectionedEndpoint{
+		baseEndpoint: baseEndpoint{Queue: queue, receiver: receiver, connected: connected},
+		id:           uid.UniqueID(),
+		idGenerator:  uid,
+		stype:        stype,
+	}
+}
+
 // ID implements ConnectingEndpoint.ID.
 func (e *connectionedEndpoint) ID() uint64 {
 	return e.id